1. Field of the Invention
This invention relates to a device for optically determining the angle of rotation of a rotating member.
2. The Prior Art
Applications are known where it is desirable to determine the angle of rotation of a rotating member. For example, in printing devices with moving print heads, the position of the print head along the width of the paper may be set or determined by measuring the angle of rotation of the driving shaft of an electric motor. In the past, this has been accomplished by the employment of a transmission light sensor comprising a light emitter and a light receiver between which is disposed a perforated disk corotatable with the shaft of the rotating element. When the perforated disk is placed in rotation, the constant light stream emitted by the light emitter is periodically interrupted. These interruptions are evaluated at the light receiver and, under certain conditions, are supplied to a signal converter. The signals thus obtained provide a means for measuring the angle of rotation, as well as the speed, of the rotating element.
Structural constraints have imposed limits on the resolution of such prior art devices. For example, the diameter of the perforated disk cannot be made arbitrarily large. Furthermore, the aperture width of the light receiver may not be made too small. These structural constraints as well as the desire for maximum resolution of the angle of rotation measurement necessitate that the width of the light permeable and of the light impermeable areas of the perforated disk must be significantly larger than the aperture width. As a result, a limit is placed on the fineness of the perforated disk division which, especially with small diameter perforated disks, allows only a coarse resolution of the angle of rotation.
One approach to increasing the resolution of such prior art devices is to increase the diameter of the perforated disk. However, it is often the case that the diameter cannot be made as large as is necessary. Another approach is to increase the number of perforation divisions; but as the width of the light permeable areas of the perforated disk approach the width of the light receiving aperture, the shape of the output signal becomes asymmetrical, thus effecting an increasing error in the rotation angle measurement. This result is particularly disadvantageous when the rotating element is the shaft of a DC motor used in printing or writing devices for driving a carriage, since every deviation from the prescribed angle of rotation will appear as a position error of the carriage.